Cryogenic fluids such as liquid hydrogen (LH2) and liquid oxygen (LO2) are well-suited for use as propellants for extended-duration space missions due to their relatively high energy content per unit mass. Cryogenic propellants are also desirable for missions wherein the spacecraft must achieve relatively large changes in velocity to enable the spacecraft to reach a given orbit or attain a desired trajectory. A cryogenically-fueled rocket engine additionally provides the capability for engine shut-down and restart as may be required multiple times over the course of a mission. Even further, certain cryogenically-fueled rocket engines may be throttled to provide different levels of thrust at different times during a mission.
Advantageously, cryogenic fluid tanks require a relatively low amount of internal pressurization to prevent vaporization of the liquid cryogenic fluid. However, the tank internal pressure must be controlled to maintain the tank within its structural limits. In the environment of space, the tank internal pressure generally increases as a result of heating of the tank exterior from radiation and from other heat sources. The heating of the tank continuously warms the cryogenic fluid resulting in boil-off wherein a portion of the liquid cryogen is vaporized into the gaseous phase. The constant boil-off and pressure buildup within the tank may be controlled by minimizing temperature increases in the cryogenic fluid and by venting the tank.
Ideally, in a cryogenic propulsion system, only the gaseous phase of the cryogenic propellant is vented from the tank in order to maximize the reduction of tank internal pressure and avoid expulsion of valuable liquid propellant. In a significant gravitational environment such as on Earth or in response to the acceleration of a firing rocket engine, the high-density liquid phase of the cryogenic fluid is pulled toward the aft end of the tank near the engine feed line. Under such conditions, the low-density gaseous phase is located above the liquid phase at a forward end of the tank and can be directly vented to the tank exterior.
However, in the low-gravity environment of space, the liquid and gaseous phases of the cryogenic propellant are free to move about the tank. Direct venting from the forward end of the tank may result in expulsion of liquid phase with the gaseous phase. Furthermore, when the vehicle is in coast mode, deceleration forces on the vehicle due to on-orbit drag may cause the liquid propellant to migrate toward the forward end of the tank preventing direct venting of the gaseous phase from the tank forward end.
As can be seen, there exists a need in the art for a system and method for reliably venting the gaseous phase of fluid contained within a cryogenic tank in a low-gravity environment to prevent over-pressurization of the tank. Furthermore, there exists a need in the art for a system and method for minimizing temperature increases of the cryogenic fluid in order to minimize increases in tank pressure. For missions of extended duration, such a system preferably minimizes boil-off of the cryogenic liquid to maximize the amount of available propellant.